Characterization of the microemulsion
The
electrical
conductivity (κ) of the microemulsion is sensitive to the change
of its microstructure.29,30 Therefore, it is usually
used to distinguish the subregions in the microemulsion. The variations
of electrical conductivity with respect to the mass fraction of oil
phase (WOA, octane phase containing 50 mmol
L-1 acetic acid) at different mass ratios ofn-propanol to water (RP/W) are presented in
Figure S1 and some typical results are shown in Figure 2A. It can be
found that the κ value
increases initially and then decreases with the increasing of
WOA value. For the purpose of clarity, the curve ofκ with the increase of WOA at fixed
RP/W of 9.02 is taken as an example alone (Figure 2B).
Obviously, there are two breaks at the tested WOA values
of 10.4 and 20.8 wt.%, which divide the curve into three stages.
At the first stage, the initial increase of κ before the
WOA value reaching to 10.4 wt.% is attributed to
the successive increase of the
conductivity of oil/water (O/W)
microemulsion droplets. This behavior can be further assigned to the
continuous adding of octane into aqueous phase since the droplets can
absorb the ions from water and the charges are generated by friction in
the aqueous phase. Thus, the larger conductivity value with the
increased WOA indicates the formation of O/W
microemulsion. Then, the electrical
conductivity begins to decrease with further increase of
WOA, showing the generation of
bicontinuous (B.C) microemulsion,
where the O/W microemulsion droplets are broken by collision, and the
oil phase also begins to aggregate to form network-like structures,
which can be observed distinctly by the following Cryo-SEM image (Figure
5B). In this stage, an inverse relationship between electrical
conductivity and octane concentration is observed. It is because the
conductivity of this system depends on the conductive channels generated
by the continuous water phase, where these channels became much narrower
with the decline of water content. When the WOA value
exceeds 20.8 wt.%, the κ value decreases linearly with
the increase of WOA, which corresponds to the formation
of water/oil (W/O) microemulsion. In this W/O microemulsion region, the
continuous phase is octane and the κ value only relies on the
movability of water micro-droplets, where the gradual disappearance of
water droplets results in a linear decrease of κ value in this
system with increasing WOA (more than 20.8wt.%). Thus, the three subregions marked O/W, B.C and W/O of the
single-phase region shown in Figure 1 are distinguished clearly through
electrical conductivity measurements.